Sound recording system



June 29; 1943." HAC 1' 2,323,032

- scum: RECORDING "SYSTEM Filed ll ay 24,1941 2 Sheets-Sheet 1 P1 7; -Ca

Fig.3

v lnventor: A E HACK 2 "'1 L WAUMM A TTOR/VEV June 29, 1943. 2 E. HACK2,323,032

2 SOUND RECORDING SYSTEM Fil ed May 24, 1941 w 2 Sheets-Sheet 2 Fig.5

Inventor:

E HACK %uwfwmw A Tram/5r Patented June 29, 1943 SOUND RECORDING SYSTEMErnst Hack, Berlin-Charlottenburg, Germany; vested in the Alien PropertyCustodian Application May 24, 1941, Serial No. 395,014. In

Spain, Switzerland, Sweden, Italy, the Netherlands, and Denmark July 10,1939 3 Claims.

This invention relates to sound recording systems and particularly tosuch systems wherein the signal variations and variations according tothe envelope of said signal variations are impressed upon the film.These systems include noise reduction and volume range compressionsystems.

Such systems comprise two channels, one channel for the signalvariations and a special channel for the envelope variations. Theenvelope variations may operate a swinging element e. g. a shutter.According to a patent application previously filed not only shall thisswinging element be operated by the special channel according to theenvelope variations-but also it shall get an impact or rapidly appliedvoltage by a second special channel.

According to the present invention this second special channel is to beblocked after arrival of said electrical impact.

According to another feature of the invention the blocking is producedautomatically and for example preferably by a change of voltageimpressed with retardation from the first of said special channels upona member in the second channel. The blocking of the second channel thusoccurs practically before the control energy of the first channelarrives at the control element.

Further features of the method as well as circuit diagrams to practicesaid method will be evident from the following description and thedrawings.

Fig. 1 shows a circuit according to the invention.

Figs. 2 and 3 show circuits according to the invention. The circuitdiagrams show only the circuit for controlling the shutter for noisereduction.

Figs. 4 and 5 show a sound record according to the invention.

Of the microphone current which in known manner is supplied to theoscillatory element (for example an oscillograph mirror) a part isbranched off and applied to the input-terminals a and D. From here thecurrent runs (Fig. 1) through potentiometers P1 and P2 connected inparallel relationship. By means of potentiometer P1 a higher Voltage isapplied to the primary winding of transformer T1, than the voltageapplied to the primary terminals of transformer T2 through potentiometerP2. The secondary windings of the transformers T1 and T2 are connectedto the plates of the rectifiers D1 and D2. The cathode of rectifier D1is connected in the usual manner with thegrid of the amplifier tube Vthrough resistances R3 and R4. The centertaps of the secondary windingsof transformers T1 and T2 are joined together and connected with thenegative pole of the plate voltage supply, which is connected with thecathode of amplifier tube V through resistance R11 and cathoderesistance Rs with parallel condenser C4. Condenser C1 serves as loadingcondenser for rectifier D1 and will be discharged through shunt resistorR2. The cathode of rectifier D2 is connected with the grid of amplifiertube V by a condenser C9 The condenser C3 serves as loading condenserfor rectifier D2 and will be discharged by the resistors R1 and R2. Thetime constant for the rectifier circuit D1 results is determined by thelinks R2-C1, R3--C2 and R4 with C9 and C3 connected in series, while thetime constant for the rec tifier D2 is determined by R1--C3 and R-1-C2.It may be remarked that the condensers Ca and C9 are small compared withthe other condensers.

The circuit works in the following way:

If a sinusoidal wave arrives at the terminals a and b a certain part ofit flows to rectifier D1 through potentiometer P1 and a smaller amountto rectifier D2 through potentiometer P2. The voltage at D2 is appliedby condenser C9 directly to the grid of amplifier tube V in which platecircuit the shutter is located. Thus the comparatively small condenserC3 is charged. The current rectified through rectifier D1 charges firstthe condenser C1, then through resistances R3 and R4 the condenser C2and at last the combination of C9 and C3; then it reaches thegrid of theamplifier tube V. Therefore the rectified currents of the rectifiers D2and D1 reach the control grid one after the other. In such way theshutter receives first through rectifier D2 an impact of high currentwhich forces it to open very fast. Thereafter the rectifier D2 will beblocked in a time determined by the internal resistance of rectifier D1and the value of C1. The voltage drop across the resistance R2, due tothe current from the rectifier D1, causes a current to fiow through theresistance R1 which is opposite in direction to the current from therectifier D2, and, as the voltage from transformer T1 is larger than thevoltage from transformer T2, the voltage drop in the resistance R1 dueto the current from rectifier D1 is larger than the voltage drop due tothe current from rectifier D2, thus impressing on the cathode of D2 avoltage which blocks the rectifier D2. This prevents the rectifier D2from acting further upon the tube V and also prevents unquiet movementsof the shutter.

In order to prevent the shutter swinging beyond the desired limitsthrough the acceleration which is produced by the impact of currentdelivered by rectifier D2, the ratio of time constants betweenrectifiers D2 and D1 is computed in such a way that after decaying ofthe first impact of current a certain time passes until the arrival ofthe impulse from rectifier D1 through the links R3-Cz and R4-C9--C3;during this time the acceleration ceases and the shutter is, through thecurrent delivered by rectifier D1 kept open in approximately theposition which has been reached through the impact of current fromrectifier D2.

On account of the inertia of the shutter construction, it may bedesirable to have the change in the anode current of the amplifier tubeV due to the current from the rectifier D2 relatively greater than thechange in the anode CUT".

rent due to the current from the rectifier D1. In the absence of asignal current, the anode current of the amplifier tube V holds thenoise reduction shutter closed. Upon the application of a signal to theterminals a, b, the rectifier D2 3 creased voltage drop from anode tocathode decreases the voltage applied to the condenser G4 whichpartially discharges through the resistor R5 increasing the negativevoltage on the grid of the amplifying tube V and accentuating thedecrease in the anode current. The cathode re-- sistor R5 and condenserC4 are designed to have such a time constant that the plate current ofthe amplifying tube V restores at about the same rate as the blockingvoltage is applied through the resistance R1 to the rectifier D2. Theresistor R11 serves to compensate the inverse current of rectifier D2and D1.

Fig. 2 shows a similar circuit-diagram. The rectifiers D1 and D2 arehere replaced by a single tube D embodying two diodes which are suppliedwith different voltages from a common transformer T3. The amplifier tubeV comprises two systems; the shutter lies in a bridge circuit betweenthe two plates in order to reduce the effect of the plate voltagefluctuations upon the shutter. The operation of the circuit issubstantially the same as the circuit of Fig. 1.

In Fig. 3 the shutter is placed directly into-the direct-current circuitof the rectifier tube. In order to obtain the decrease of amplification,de-

scribed in Fig. 1, a tube H with variable mu characteristics isconnected ahead of rectifier D3. The control grid of said tube receivesthrough a potentiometer P3 located in the direct current circuit avoltage depending always of the direct current, said voltage decreasingthe amplification of tube H according to its variable mu characteristicswith increasing input voltage on terminals 9 and 6.

Fig. 4 shows a record made by using a circuit according to Fig. 1. It isnot essential that the impact supplied by rectifier D2 acts upon theelement which is operated by the rectified energy originating fromrectifier D1; but it can also act upon the member upon which areimpressed the signal currents, ,for example upon an oscillograph mirror.A record according to Fig. 5 will be the result.

I claim as my invention:

1. In a system for controlling a recording device, two control channelshaving a common input for modulated electrical energy and a commonoutput to a recording device, the energy supplied to the first channelbeing less than the energy supplied to the second channel, each channelincluding a rectifier and a smoothing network, the time constant of thenetwork in the first channel being less than the time constant of thenetwork in the second channel, and an element common to both networks,whereby the potential developed across said element by the current fromsaid second rectifier reduces the current from said first rectifieruntil said rectifier is blocked.

2. In a control network having an input and an output circuit, tworectifiers connected to said input circuit, a load circuit for one ofsaid rectifiers including two impedance elements in serial relationshipconnected to said output circuit, and a load circuit for the otherrectifier including one of said elements.

3. In a control network having an input circuit and an output circuit,two rectifiers connected to said input circuit, two resistors in serialrelationship connected in series with one of said rectifiers, one freeend of said resistors being connected to the output circuit, a firstcapacitor connected from the other free end of said resistors to saidoutput circuit, a second capacitor connected in parallel relationshipwith both said resistors, means for connecting one of said resistors inserial relationship with the other rectifier, and a third capacitorconnected in parallel relationship with said latter resistor.

ERNST HACK.

